The subject of the invention is a stopping device for a single-phase asynchronous motor with phase shifting capacitor using the variation in the voltage across the terminals of the phase shifting capacitor as a function of the motor speed itself varying with load, so as to stop the motor when the load on the motor exceeds a specified value.
Such a device is described in patent FR 2 649 260. This device comprises a circuit delivering a DC voltage homothetic to the voltage across the terminals of the phase shifting capacitor of the motor, a circuit delivering at least one constant reference voltage and a comparator circuit comparing these two voltages and delivering a stop signal when the homothetic DC voltage becomes equal to or less than the reference voltage.
Such a circuit is especially useful as safety device for stopping the motor of an installation in which the load driven by the motor is at risk of encountering an obstacle with the attendant risks of damage or impairment of the installation. Motorized shutters, blinds and doors may be mentioned by way of example.
A similar device is described in patent EP 0 551 053. This device furthermore comprises means of compensation for taking account of the fluctuation of the voltage across the terminals of the phase shifting capacitor with the temperature of the motor. In the above device the reference voltage consists preferably of a voltage slaved to the voltage of the power supply so as to circumvent fluctuations of this voltage.
In the two devices according to the prior art, the measured voltage is applied to a transformer followed by a rectifier circuit before being applied to one of the inputs of a comparator circuit, the transformation ratio being substantially equal to 30.
Now, in practice, such devices may prove to be incapable of detecting a variation in torque with satisfactory sensitivity and precision. This stems from the fact that in an asynchronous motor with phase shifting capacitor, the voltage across the terminals of the capacitor is substantially constant regardless of the characteristic of the motor. This voltage is substantially 560V when the motor is not subjected to any load and 460V when the motor is locked, that is to say loaded to its maximum torque. The total variation in the voltage across the terminals of the capacitor is therefore 100V and this variation is substantially linear. Thus, if one wishes to detect a variation in torque of 1N, for a motor developing a maximum torque of 4N, the variation in voltage across the terminals of the capacitor will be 100V/4, that is to say 25V, while for a motor developing a maximum torque of 40N the variation in voltage across the terminals of the capacitor will be 100V/40, that is to say 2.5V.
Considering the transformation ratio of the transformer to be 30, one therefore obtains, across the terminals of the capacitor, for a torque variation of 1N a voltage variation substantially equal to 25V/30=0.8V for a motor developing a torque of 4N and a voltage variation substantially equal to 2.5V/30=0.08V for a motor developing a torque of 40N. This latter value may prove to be totally insufficient to detect with satisfactory sensitivity and precision a rise in torque which must necessarily cause the stoppage of the motor.
The aim of the invention is to remedy this drawback.
The stopping device according to the invention is characterized in that it comprises means for transforming the variation in the voltage across the terminals of the phase shifting capacitor corresponding to a torque variation of 1N into a chosen variation of the voltage regardless of the maximum power of the motor, means for comparing this chosen variation with a reference voltage and means for stopping the motor when the transformed voltage is less than the reference voltage.
According to one mode of execution of the invention, the means for transforming the voltage comprise a voltage rectifier, a voltage reducer and a voltage divider to which is applied the voltage obtained after the voltage reducer.
The voltage reducer preferably consists of a Zener diode, which constitutes the ideal means of achieving a voltage drop. The voltage drop could however be obtained by means of an ad hoc circuit.
The divided voltage can be processed by means of an analog circuit or, by sampling, by means of a microprocessor.
For a torque variation of 1N, a voltage variation matched to the processing circuit used and ensuring good sensitivity will be chosen. The only components of the device which have to be matched to the motor are the Zener diode and the value of the resistors of the voltage divider, or even just the value of one of the resistors.